Specifications

Chapter 4 - Transient Protection of RS-422 and RS-485 Systems RS-422 And RS-485 Applications Ebook
Page 58 of 137 Manual Documentation Number: <DocNumber>
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Transient Protection using Shunting
Shunting Theory
Creating one common ground at the host device provides a safe place
to divert surge energy as well as a voltage reference to attach surge
suppression devices to. Shunting harmful currents to ground before
they reach the data port is the job of components such as transient
voltage suppressors (TVS), metal oxide varistors (MOV) or gas
discharge tubes (GDT). These devices all work by "clamping" at a set
voltage. Once the clamp voltage has been exceeded, the devices
provide a low impedance connection between their terminals.
Figure 43: Transient Suppressor Symbols
Since this type of device diverts a large amount of energy, it cannot
tolerate long durations or continuous transients. Shunting devices are
most often installed from each data line to the local earth ground, and
should be selected to begin conducting current at a voltage as close as
possible above the system's normal communications levels. For RS-422
and RS-485 systems, the voltage rating selected is typically 6 to 8
volts. These devices typically add some capacitive load to the data
lines. This should be considered when designing a system.
Compensation for the added capacitance can be accomplished by
derating the total line length. Several hundred feet is usually adequate.
Transient suppressors should be installed as close as possible to the
port to be protected, and the user must provide an extremely low
impedance connection to the local earth ground of the unit being